Potassium (given the symbol "K") is one of the macrominerals that your horse needs.
It is one of the most important minerals throughout the body.
As the major intracellular cation (an ion with a positive charge that is found inside cells), it plays a huge role in maintaining acid-base balance.
It is also essential in maintaining osmotic balance...or the amount of water that is inside cells.
However, its biggest job is to be involved in skeletal muscle excitability through potassium ion channels.
Approximately 75% of this mineral in the equine body is found in the skeletal muscle.
These channels are located throughout your horse's body in cell membranes. They are opened in various ways (by voltage, presence of calcium, etc.) to allow K either in or out of cells.
In "excitable cells" (for instance, neurons, which make up nerves), K is responsible for exciting the cell. For instance, when your horse wants to swish his tail to get rid of a fly, the muscles attached to the tail bone must be excited so that they contract. They are excited by potassium being rushed across cell membranes. So, every time your horse moves a muscle he's using K. The heart is also a muscle...and one that is controlled by K. The amount that moves across the heart's potassium ion channels can be increased or decreased to speed up or slow down the heartbeat.
Not only are these channels essential in excitable cells, they are also important in other cells because they regulate the release of hormones. If there is a malfunction of the channels, some hormones, such as insulin, may be mis-regulated.
Most forages contain a large amount of K...1-2% on a dry matter basis. Cereal grains, on the other hand, contain only 0.3-0.4% on a dry matter basis.
Due to the large amount of forage that horses consume, their daily intake usually is much more than their requirement. Therefore supplementation is very rarely needed.
Despite the large excess of intake, this excess appears to be of little concern. The equine kidney is very efficient at removing excess K from the body. When potassium intake is increased, urinary excretion increases, followed by fecal excretion.
It has also been found that horses will refuse to eat if given a diet with excess K and deprived of adequate water. This mechanism ensures that toxicity is of very little concern...
As long as there is water available to the body, potassium can be excreted through urine...if the horse only takes in K when he has water available, he'll never run into a situation where he has an excess and no ability to excrete it through urine.
Most horses will not have to worry about a deficiency. When foals were fed potassium-deficient diets in one study (which usually doesn't happen in practical horse-keeping situations), they eventually stop eating, and as a result become very poor looking. However, when the foals were given supplemental K in addition to their deficient diet, they immediately started eating again, and their appearance improved greatly.
However, horses that are exercising heavily may develop a deficiency, especially in hot and humid weather where they sweat more and therefore lose more electrolytes (potassium is a major electrolyte). Up to 10 to 15 L of fluid can be lost per hour in exercising horses. Horses that are exercising for prolonged periods (for example, an endurance horse) are at particular risk. The administration of IV or oral electrolytes may not be enough to correct the loss from sweat, as these products are higher in sodium than K, while the sweating horse loses more potassium than sodium. However, due to the excess dietary intake that is common, K may not even need to be supplemented at all.
The most common time that supplementation may be necessary is for very strenuous events (especially in hot climates) where the horse is exercising for long periods without a chance to eat.